There has been offered an image display device that uses a light emitting diode (LED) as a light emitting element. The image display device of this type is configured by including a plurality of LEDs for respectively emitting lights of red (R), green (G), and blue (B) colors, an illumination optical system into which the lights from the plurality of LEDs enter, a light bulb including a liquid crystal display panel into which the lights from the illumination optical system enter, a color synthesis prism for synthesizing the lights of the respective colors from the light bulb, and a projection optical system for projecting the lights from the color synthesis prism onto a projection screen.
In the image display device thus configured, to increase the luminance of a projected image, it is required to prevent light loss as much as possible in an optical path from the light emitting element to the projection optical system.
Among the aforementioned components, the liquid crystal display panel and the color synthesis prism are polarization-dependent. To achieve high efficiency of the optical system, therefore, the light emitting element advisably emits linearly polarized light.
As the light emitting element for emitting linearly polarized light, a light emitting element that includes a polarizer, a wavelength plate, and a reflection layer can be employed.
As a light emitting element that includes the polarizer, the wavelength plate, and the reflection layer, there is an element that reflects the light reflected by the polarizer on the reflection layer via the wavelength plate, and that changes a polarizing direction again via the wavelength plate to enter the light into the polarizer, and an element that enters the light into first and second polarizers through which polarized lights orthogonal to each other are passed, that provides a phase difference to the light passed through one of the polarizers by the wavelength plate to change the polarizing direction, and accordingly that emits the lights passed through the first and second polarizers as linearly polarized lights made uniform in a polarizing direction.
In the light emitting element thus configured, the wavelength plate for changing the polarizing direction is important. Generally, the wavelength plate has a narrow permissible range of incident angles to function as the wavelength plate, and thus widening the incident angle permissible range of the wavelength is one of the important factors for improving the performance of the light emitting element.
Patent Literature 1 (JP2001-51122A) discloses, for the purpose of providing a birefringent element tat is industrially manufacturable at low costs, and that exhibits large birefringence having an optical axis within a plane, and is permitted a large opening area with a small optical path length, a periodic structure that is a z-axis direction multilayer structure including two or more types of transparent members whose refractive index in three dimensional orthogonal coordinates x, y and z: a shape of a layer as a stacking unit for each transparent member having a periodic concave-convex structure in an x axis direction and a periodic or nonperiodic concave-convex structure of a length uniform or larger than that in the x axis direction and in a y axis direction, and layers being stacked in a z axis direction by repeating the shape at each period shorter than a first Bragg condition in the z direction. By entering light to this periodic structure vertically or obliquely to an xy surface, and providing a phase difference between a polarized wave having an electric field component in the x direction and a polarized wave having an electric field component in the y direction, the periodic structure is used as a phase plate.
Patent Literature 2 (WO2004-113974A1) discloses, for the purpose of expanding the operation band of a polarization separation element and increasing incident angle tolerance, a configuration for gradually changing a stacking period in the z direction with respect to the configuration disclosed in Patent Literature 1.